Infrared laser

ABSTRACT

Laser devices are presented in which a graphene saturable absorber and an optical amplifier are disposed in a resonant optical cavity with an optical or electrical pump providing energy to the optical amplifier.

REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of U.S. ProvisionalPatent Application Ser. No. 61/559,308, filed Nov. 14, 2011, andentitled “Method for Implementing Infrared Laser”, the entirety of whichis hereby incorporated by reference.

BACKGROUND

Laser devices provide output from an optical cavity when an includedgain medium overcomes the cavity losses so that amplification viastimulated emission occurs. Lasers produce light over some naturalbandwidth or range of frequencies, sometimes referred to as the gainbandwidth, which is largely determined by the laser gain medium and theoptical cavity or resonant cavity of the laser. Saturable absorbers canbe introduced into the optical cavity to provide variability in thecavity loss, which is typically nonlinear such that when a small numberof photons are present there is a larger cavity loss than when a largenumber of photons are present. This allows Q-switching to occur if theexcited upper state lifetime of the gain medium is sufficiently long toaccumulate enough energy to overcome the cavity losses by forming apulse. Alternatively, if the gain bandwidth and laser cavitysimultaneously support a large optical bandwidth, a saturable absorbercan initiate mode-locking or phase-locking of the laser by providinglower loss to a multiple longitudinal optical modes with a specificphase relationship within the laser resonant cavity. Mode-locked laserscan produce extremely short duration light pulses, on the order ofpicoseconds or femtoseconds. Moreover, passive, hybrid, or activemode-locking can be produced in a laser. Active mode-locking can beaccomplished using an external signal to induce intra-cavity lightmodulation. Passive mode-locking, in contrast, involves incorporation ofa structural element, such as a saturable absorber into the laser cavityto cause self-modulation of the light. In this manner, passivemode-locked lasers use the light in the cavity to cause a change in someintra-cavity element, which will then itself produce a change in theintra-cavity light. Saturable absorbers exhibit intensity-dependenttransmission or reflection, and thus behave differently depending on theintensity of the light. For passive mode-locking in a laser, an idealsaturable absorber selectively absorbs low-intensity light, whiletransmitting or reflecting light which is of sufficiently highintensity, thereby acting as an optical gate. Thusfar, implementingpassive mode-locked or Q-switched infrared lasers has been difficult,especially at infrared wavelengths, and a need remains for improvedlaser devices and implementation approaches, particularly for compactlaser device structures.

SUMMARY OF DISCLOSURE

Various details of the present disclosure are hereinafter summarized tofacilitate a basic understanding, where this summary is not an extensiveoverview of the disclosure, and is intended neither to identify certainelements of the disclosure, nor to delineate the scope thereof. Rather,the primary purpose of this summary is to present some concepts of thedisclosure in a simplified form prior to the more detailed descriptionthat is presented hereinafter.

Linear cavity, external cavity, extended cavity, and ring cavity laserdevices and implementation approaches are provided in which a fixed orposition-adjustable graphene saturable absorber structure, operating inthe reflective or transmissive mode, is situated in the laser opticalcavity, and various embodiments facilitate mode-locked or Q-switchedoperation at ultraviolet, visible, and/or infrared wavelengths. Invarious implementations, mode-locked lasers or Q-switched lasers employsemiconductor optical amplifiers (SOA) as the gain medium in combinationwith graphene saturable absorbers in a linear cavity or external cavityarrangement. In various implementations, mode-locked lasers orQ-switched lasers employ fiber or bulk crystal optical amplifiers (OA)as the gain medium in combination with graphene saturable absorbers in alinear cavity or external cavity arrangement. In certain embodiments,the graphene saturable absorber is integrated with the semiconductoroptical amplifier or fiber or bulk crystal optical amplifier. Certainembodiments, moreover, provide laser implementations by integration of agraphene saturable absorber into a vertical cavity surface emittinglaser (VCSEL) structure, integration of graphene saturable absorber intoa vertical external cavity surface emitting (VECSEL) structure, and thecombination of a graphene saturable absorber into an external cavitylaser.

In accordance with one or more aspects of the present disclosure, alaser device is provided which includes first and second mirrors (flator curved) with corresponding reflective surfaces that face one anotherto define a resonant linear optical cavity in which electromagneticenergy is amplified by stimulated emission of coherent radiation that ispartially transmitted through the partially reflective second mirror(output coupler). A semiconductor optical amplifier and a graphenesaturable absorber are disposed between the reflective surfaces and anelectrical or optical pump provides current and/or light to thesemiconductor optical amplifier. In certain embodiments, the device is aQ-switched laser or a mode-locked laser operable at center wavelengthsof 1,800 nm or more and about 200 μm (1.5 terahertz) or less, and in themode-locked case with a spectral width (spectral bandwidth) of as muchas 10% or more of the center wavelength. In certain embodiments, thespectral widths can be more than 20% of the center wavelength. Invarious embodiments, moreover, a mode-locked or Q-switched laser deviceis provided which is operable at center wavelengths of about 280 nm ormore and about 1,800 nm. Certain non-limiting embodiments provide asemiconductor optical amplifier comprising a periodic structure orchirped structure or graded structure of alternating superlattice with aperiodic series of semiconductor layers of at least two differentmaterial compositions. Certain non-limiting embodiments provide asemiconductor optical amplifier comprising a double quantum wellsuperlattice with two types of superlattice structures.

In certain non-limiting embodiments, the graphene saturable absorber isdisposed between the reflective surface of the first mirror and thesemiconductor optical amplifier. In certain non-limiting embodiments,the graphene saturable absorber is integral with the first mirror and isbetween the reflective surface of first mirror and the semiconductoroptical amplifier. In certain non-limiting embodiments, the graphenesaturable absorber is disposed between the semiconductor opticalamplifier and the reflective surface of the second mirror. In certainnon-limiting embodiments, the graphene saturable absorber is integralwith the second mirror and is between the semiconductor opticalamplifiers and reflective surface of the second mirror. The graphenesaturable absorber may be integral to the semiconductor opticalamplifier in certain embodiments, and the second mirror (the outputcoupler) may be integral to the semiconductor optical amplifier or maybe spaced therefrom. In some embodiments, the second mirror (outputcoupler) can be a partially reflective facet on a second end of thesemiconductor optical amplifier, or the second mirror may be a partiallyreflective coating on the second end of the semiconductor opticalamplifier. In certain embodiments, the second mirror (output coupler) isspaced from the semiconductor optical amplifier, and may be a Braggmirror. One or more optical components, such as a gas cell may belongitudinally disposed between the second end of the semiconductoroptical amplifier and the second mirror in certain non-limitingembodiments.

In some embodiments, moreover, the graphene saturable absorber may beintegral to the first mirror, where the first mirror can be a highlyreflective coating on the graphene saturable absorber, or the firstmirror can be a Bragg mirror (distributed Bragg reflector) or a highlyreflective coating on a substrate or a metal substrate in certainnon-limiting embodiments.

Certain embodiments of the laser device provide a graphene saturableabsorber which is spaced from the semiconductor optical amplifier, andmay include a gas cell, bandpass and other spectral filters, an etalon,a microelectromechanical (MEM) controlled etalon, polarizer, prisms,dispersive elements, lenses, or other optical component or componentsbetween the first end of the semiconductor optical amplifier and thegraphene saturable absorber. For example, the graphene saturableabsorber may be integral to the first mirror. In other embodiments, oneor more optical components may be longitudinally disposed between thesecond end of the semiconductor optical amplifier and the second mirror.

In certain embodiments, the graphene saturable absorber is spaced fromthe first mirror, may be integral to the semiconductor opticalamplifier, may be between the semiconductor optical amplifier and thesecond mirror (output coupler), or may be integral to the second mirror.One or more optical components, such as a gas cell may be located withinthe optical cavity and for example, may be disposed between the graphenesaturable absorber and the first mirror.

In certain embodiments mirrors may not be discrete objects, but stillconfine the light within a resonant cavity, e.g. whispering-gallerywaves, and the graphene saturable is integral with the cavity by director evanescent coupling with the resonant cavity.

In accordance with further aspects of the present disclosure, a linearcavity laser device is provided, which includes a first mirror with afirst reflective surface, as well as a second mirror with a secondreflective surface facing the reflective surface of the first mirror.The mirrors define a linear (longitudinal) resonant optical cavity, witha semiconductor optical amplifier disposed at least partially betweenthe first and second reflective surfaces and an electrical or opticalpump providing current or light to the semiconductor optical amplifier.In addition, a graphene saturable absorber is integral with the secondmirror at least partially between the semiconductor optical amplifierand the second reflective surface. In certain embodiments, the graphenesaturable absorber is spaced from the semiconductor optical amplifier.The semiconductor optical amplifier, moreover, can be a vertical cavitysurface emitter laser (VCSEL) in certain non-limiting embodiments. Thesemiconductor optical amplifier in certain embodiments has first andsecond ends, with the graphene saturable absorber bonded to the secondend, and the second mirror can be integral with the graphene saturableabsorber to provide a monolithic laser structure.

A linear cavity laser device is provided in accordance with furtheraspects of the present disclosure, including first and second mirrorswith corresponding reflective surfaces, along with a solid stateoptically pumped optical gain medium disposed at least partially betweenthe reflective surfaces and an optical pump providing light to thesolid-state optically pumped optical gain medium. In certainembodiments, the solid-state optically pumped optical gain medium can bean infrared (including near infrared or mid infrared) solid-stateoptically pumped optical gain medium. A graphene saturable absorber isdisposed at least partially between the solid-state optically pumpedoptical gain medium and the first reflective surface. In certainembodiments, the optical gain medium is a rare earth doped fiberamplifier operable at wavelengths of about 1.4 μm or more and about 3.0μm or less. In other embodiments, the solid-state optical gain mediumincludes at least one II-VI family doped crystal. In furtherembodiments, the solid-state optical gain medium includes at least onetransition metal doped zinc chalcogenide, or may include at least onelead salt. In certain embodiments, moreover, at least one opticalcomponent, such as a gas cell, may be longitudinally disposed betweenthe solid-state optical gain medium and the second mirror.

In accordance with further aspects of the disclosure, a linear cavitylaser device is provided, which includes first and second partiallyreflective mirrors with corresponding reflective surfaces that define alongitudinal injection locked optical cavity, together with anelectrically or optically pumped optical gain medium between thereflective surfaces, and a graphene saturable absorber disposed at leastpartially between the optical gain medium and the first reflectivesurface. A laser is disposed on a second side of the first partiallyreflective mirror to direct light at least partially toward the firstpartially reflective mirror. The linear cavity laser device may furtherinclude one or more gas cells or other optical components longitudinallydisposed between the optical gain medium and the second mirror.

A ring cavity laser device is provided in accordance with furtheraspects of the disclosure, including first and second mirrors withcorresponding reflective surfaces, as well as a third mirror with athird reflective surface, where the mirrors define a resonant opticalcavity in which electromagnetic energy is amplified by stimulatedemission of coherent radiation that is partially transmitted through thesecond mirror. An electrically or optically pumped optical gain mediumis disposed at least partially between the first and second mirrors, anda graphene saturable absorber is disposed at least partially between thesecond and third reflective surfaces.

In various implementations, linear and/or ring cavities can be employedwith more than two mirrors, for instance, to fold the cavity. Also,other optical elements (e.g., prisms, polarizers, dispersive elements,bandpass and other spectral filters, etc.) can be placed within thelaser cavity.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and drawings set forth certain illustrativeimplementations of the disclosure in detail, which are indicative ofseveral exemplary ways in which the various principles of the disclosuremay be carried out. The illustrated examples, however, are notexhaustive of the many possible embodiments of the disclosure. Otherobjects, advantages and novel features of the disclosure will be setforth in the following detailed description when considered inconjunction with the drawings, in which:

FIG. 1 is a partial sectional side elevation view illustrating a linearcavity laser device with a graphene saturable absorber and reflectivemirror attached to a first facet end of a semiconductor opticalamplifier;

FIG. 2A is a partial sectional side elevation view illustrating a P-Nquantum cascade superlattice semiconductor optical amplifier;

FIG. 2B is a partial sectional side elevation view illustrating an N-Nquantum cascade superlattice semiconductor optical amplifier;

FIG. 3 is a partial sectional side elevation view illustrating a linearcavity laser device with a graphene saturable absorber and reflectivemirror attached to a first facet end of a semiconductor opticalamplifier, as well as a partially reflective mirror spaced from thesecond facet end of the semiconductor optical amplifier;

FIG. 4 is a partial sectional side elevation view illustrating a linearcavity laser device with a graphene saturable absorber and a gas cell orother optical component disposed between the second end of thesemiconductor optical amplifier and the partially reflective mirror;

FIG. 5 is a partial sectional side elevation view illustrating a linearcavity laser device with a graphene saturable absorber integral with ahighly reflective first mirror and longitudinally spaced from thesemiconductor optical amplifier, as well as a partially reflectivemirror integral with the second end of the semiconductor opticalamplifier;

FIG. 6 is a partial sectional side elevation view illustrating a linearcavity laser device with a graphene saturable absorber integral with ahighly reflective first mirror and a gas cell or other optical componentdisposed between the graphene saturable absorber and the semiconductoroptical amplifier, with a partially reflective mirror integral with thesecond end of the semiconductor optical amplifier;

FIG. 7 is a partial sectional side elevation view illustrating a linearcavity laser device with a graphene saturable absorber integral with ahighly reflective first mirror and longitudinally spaced from thesemiconductor optical amplifier, with a separate partially reflectivemirror facing the second end of the semiconductor optical amplifier;

FIG. 8 is a partial sectional side elevation view illustrating a linearcavity laser device with a graphene saturable absorber integral with ahighly reflective first mirror and an optical component disposed betweenthe graphene saturable absorber and the semiconductor optical amplifier,including a separate partially reflective mirror facing the second endof the semiconductor optical amplifier;

FIG. 9 is a partial sectional side elevation view illustrating a linearcavity laser device with a graphene saturable absorber and a gas cell orother optical component located between the second end of thesemiconductor optical amplifier and the partially reflective secondmirror;

FIG. 10 is a partial sectional side elevation view illustrating a linearcavity laser device with a graphene saturable absorber deposited orbonded onto a first facet end of the semiconductor optical amplifier, aswell as an optical component disposed between the graphene saturableabsorber and the highly reflective first mirror;

FIG. 11 is a partial sectional side elevation view illustrating a linearcavity laser device including a graphene saturable absorber and anintegrated partially reflective second mirror;

FIG. 12 is a partial sectional side elevation view illustrating a linearcavity laser device with a graphene saturable absorber bonded to thesecond facet end of the semiconductor optical amplifier and having anintegrated partially reflective second mirror;

FIG. 13 is a partial sectional side elevation view illustrating a linearcavity laser device with a graphene saturable absorber and a solid stateoptically pumped infrared optical amplifier or gain medium;

FIG. 14 is a partial sectional side elevation view illustrating a linearcavity laser device with a graphene saturable absorber and two partiallyreflective mirrors forming an injection locked (coupled) optical cavitywith a laser to direct light at least partially toward a second side ofthe first partially reflective mirror;

FIG. 15 is a partial sectional side elevation view illustrating a ringcavity laser device with two highly reflective mirrors and a partiallyreflective mirror, including an optical amplifier or gain mediumdisposed between a first one of the highly reflective mirrors and thepartially reflective mirror, and a graphene saturable absorber disposedat least partially between the second highly reflective mirror and thepartially reflective mirror;

FIG. 16 is a partial sectional side elevation view illustrating a linearcavity laser device with four mirrors and two optical elements, in whichat least one of the optical elements is a graphene saturable absorber;

FIG. 17 is a partial sectional side elevation view illustrating a ringcavity laser device with five mirrors and four optical elements, with atleast one of the optical elements being a graphene saturable absorber;and

FIGS. 18-20 are partial side elevation views illustrating furtherdetails of vertical cavity embodiments.

DETAILED DESCRIPTION OF THE DISCLOSURE

One or more embodiments or implementations are set forth in conjunctionwith the drawings, where like reference numerals refer to like elementsthroughout, and where the various features are not necessarily drawn toscale.

Semiconductor optical amplifiers for ultraviolet, visible, and nearinfrared wavelengths typically have an interband design in which photonsare emitted by the recombination of electron-hole pairs across thematerial bandgap. Semiconductor optical amplifiers for infraredwavelength can have interband cascade semiconductor optical amplifierdesign or quantum cascade semiconductor amplifier design. The interbandcascade semiconductor optical amplifier laser typically operate in thewavelength range of 3-5 μm and quantum cascade semiconductor opticalamplifier typically operate in the wavelength range of in the 5-18 μmbut terahertz lasers to 200 micron wavelength have been experimentallydemonstrated. The semiconductor optical amplifier designs typically haveantireflective coatings on their facets and also can have tilted facetsto reduce the refection of light back into the semiconductor opticalamplifier.

Single mode or multimode laser devices are disclosed herein whichinclude a graphene saturable absorber and at least two mirrors, whereinflat mirrors may be used, but alignment to a required precision may bedifficult. Transmissive saturable absorbers may be employed, andextended cavities may be provided. One or more mirror positions may beadjustable to facilitate mode-locking, and quantum cascade laser (QCL)master-oscillator power-amplifiers (MOPAs) may be included. In certainembodiments, moreover, polarization optics, and dispersive optics,spectral filters or other additional optical elements may be providedwithin the laser cavity, for instance, a MEMS controlled bandpass filterwithin the cavity. An output coupler may be provided, such as a onedimensional distributed feedback (1D-DFB), photonic crystal distributedfeedback (PCDFB), or a ring cavity surface emitting (RCSE) QCLs.Substrate emitting DFB QCL configurations are contemplated in which thefunctions of the distributed feedback and the surface emission areseparated. Certain embodiments provide substrate emitting quantumcascade ring lasers with high power single transverse mode QCLs andapplications of plasmonics to achieving high beam quality (lowdivergence, multibeam operation etc.). Two section devices arecontemplated where one section acts as the pumping section (referred toas master oscillator (MO) section) and the other section serves as apower amplifier (PA). The MO section features a DFB grating to ensuresingle-mode emission. Moreover, single mode quantum cascade lasers andplasmonic collimators are contemplated.

Mid-IR mode-locked laser may be fabricated from an interband cascadelaser (ICL) in the 3-5 μm range and quantum cascade lasers (QCL) areprovided in the 5-12 μm region. Short pulsed operation of these laserspreviously has not been achieved due to the poor energy storagecapabilities of the gain media. While this hinders pulsed operation ofthese materials via Q-switching, it does not affect the mode-lockedoperation. Producing a burst of photons by switching the gain or loss ina laser cavity requires storing energy in the laser's excited state andthen suddenly modulating this gain/loss within the cavity to produce apulse. This, however, is not the only way to produce optical pulses.Mode-locking a laser produces optical pulses by “locking” the phase of alarge number of longitudinal modes within the laser, similar in conceptto phasing a series of harmonic sine waves to produce a step. Thistheory exploits the operation of many continuous wave (cw) optical modesin the cavity by locking their phases, thereby creating an optical pulsewithout energy storage.

Referring initially to FIGS. 1-4, various non-limiting laser deviceembodiments are hereinafter illustrated and described, in which agraphene saturable absorber 130 is provided in the laser optical cavityor resonator. The type of linear cavity in certain embodiments may be a“Fabry-Perot” optical cavity. Graphene is substantially an sp2 bondedcarbon layer which forms in a two-dimensional honeycomb crystal latticelayer or sheet with a carbon-carbon bond length of about 0.142 nm.Multiple stacked sheets may form with an interplanar spacing of about0.335 nm, and may be stacked in certain arrangements such as A-B(Bernal) ordering. One exemplary linear cavity laser device 100 with afirst end 101 and a second end 102 is shown in FIG. 1, including a firsthighly reflective mirror 110 with a first reflective surface 112, and asecond partially reflective mirror 120 with a second reflective surface122 at least partially facing the first reflective surface 112 of thefirst mirror 110. The mirrors in various embodiments can be flat orcurved. The mirrors 110 and 120 define a laser optical resonator oroptical cavity with a cavity length 103 extending along a longitudinalaxis 104 in which electromagnetic energy is amplified by stimulatedemission of coherent radiation that is partially transmitted through thesecond mirror 120. Within the optical cavity is a semiconductor opticalamplifier (SOA) 140 disposed at least partially between the first andsecond reflective surfaces 112 and 122, which is provided or “pumped”with energy (e.g., electrical current or light) 152 via an electrical oroptical pump 150. The semiconductor optical amplifier in certainembodiments may have anti-reflection coatings on the facets and can havetitled facets to reduce the stimulated emission light back intosemiconductor optical amplifier. In addition, a graphene saturableabsorber 130 is disposed in the optical cavity at least partiallybetween the mirrors 110 and 120. In certain embodiments, the graphenesaturable absorber structure 130 can be an antiresonant design, or aresonant design, or a no resonant design.

Any type of semiconductor optical amplifier 140 can be used. As seen inFIGS. 2A and 2B, certain implementations of the SOA 140 include asuperlattice with a periodic series of semiconductor layers of at leasttwo different material compositions in a waveguide configuration. In theexample of FIG. 2A, the SOA 140 extends between first and second facetsor ends 141 and 142, respectively, and includes a P-N quantum cascadesuperlattice configuration, with an uppermost layer of a firstsemiconductor type (SEMICONDUCTOR 1) being heavily doped with a P typedopant (P+) and a lower most semiconductor layer of a second type(SEMICONDUCTOR 2) being N-doped (N+). The facets 141, 142 can be coatedfor antiflectivity and can be titled. In the case that the graphenesaturable absorber 130 is bonded to the first end of the SOA 140, thefacet is generally not titled in certain nonlimiting embodiments. Thefacet 141 and/or 142 can be titled for the case that the graphenesaturable absorber 130 is bonded to the second end of the SOA 140. Ohmiccontacts 144 are provided at the top and bottom of the SOA superlatticestructure 140 in the illustrated embodiment, for connection to anelectrical pump 150 and an electrical reference or ground by which theSOA 140 is pumped with electrical current 152 in certainimplementations. In other embodiments, the SOA 140 can be opticallypumped via an optical pump 150 providing light 152 to the SOA structure140. FIG. 2B shows another embodiment of the SOA 140, in which theuppermost and lower most semiconductor layers of the superlatticestructure are both doped with dopants of the same conductivity type(N+). Quantum Cascade Laser embodiments can be implemented as awaveguide on a semiconductor.

In the example of FIG. 1 (as well as the examples of FIGS. 3-10), thegraphene saturable absorber 130 is longitudinally disposed between thefirst reflective surface 112 and the SOA 140. In addition, the graphenesaturable absorber 130 in FIG. 1 is integral with the first end or facet141 of the SOA 140, and the second mirror 120 is integral with thesecond SOA end or facet 142. In certain non-limiting implementations,the second (partially reflective) mirror 120 can be a partiallyreflective facet on the second end 142 of the SOA 140. In other possibleimplementations, the second mirror 120 is formed as a partiallyreflective coating on the second end 142 of the SOA 140. In practice,the laser device 100 can be a Q-switched laser or a mode-locked laserconfigured to emit stimulated coherent radiation with center wavelengthsof about 1,800 nm or more and about 25,000 nm or less and having aspectral width as much as 10% or more of the center wavelength throughthe second mirror 120. Mode-locked laser devices 100 have been createdwith 0.006% spectral widths to 15% spectral widths. In certainembodiments, the spectral width can be greater than 0.5%, and as much as10% or more. In further nonlimiting embodiments, the spectral width isgreater than 2%. In certain embodiments, the graphene saturable absorbercan be bonded to the first end of the SOA. In other possibleimplementations, the laser 100 is a Q-switched or mode-locked device,operable to emit stimulated coherent radiation with center wavelengthsof about 280 nm or more and about 1,800 nm or less and having a spectralwidth as much as 10% or more of the center wavelength through the secondmirror 120.

Passive mode-locked or Q-switched laser operation involves an opticalgain media 140, 170, 180 that operates at the operation wavelength, aswell as an optical cavity for feedback, and a saturable absorber 130that is effective at the operational wavelength. Various non-limitingembodiments are illustrated and described below in connection with FIGS.3-15, each employing graphene saturable absorber 130 in combination withan optical gain medium such as an SOA 140 (FIGS. 1-10), a semiconductoroptical amplifier or vertical cavity surface emitter laser (VCSEL)(FIGS. 11 and 12) or other optical amplifier medium (FIGS. 13-15).

Graphene saturable absorbers 130 facilitate broadband operation throughgapless linear dispersion of Dirac electrons. The optical absorption,ultrafast carrier relaxation time, and controllable modulation depth ofgraphene have been actively measured, and the thermal conductivity ofmonolayer graphene was measured to be approximately 5000 μm⁻¹ K⁻¹ atroom temperature, which is the highest value ever found among anynatural material. In addition, several graphene mode-locked fiber lasershave been demonstrated at wavelengths near 1000 nm and 1550 nm.Wide-range wavelength tunability was also realized in both mode-lockedand Q-switched fiber lasers. In particular, the use of graphenesaturable absorbers 130 for mode-locked or Q-switched lasers 100facilitates operation at infrared wavelengths. One passive approach tomake a mode-locked laser 100 is to combine a saturable absorber withoptical gain medium in an external cavity. However, conventionalsemiconductor saturable absorbers do not operate in the infraredwavelengths. The inventors have appreciated the graphene performs wellas a saturable absorber 130 for all infrared wavelengths, and thus amode-locked or Q-switched laser 100 can be implemented at infraredwavelengths by combining the graphene saturable absorber 130 with anoptical gain medium (e.g., SOA 140 in this embodiment). Graphene'sperformance as saturable absorber is different from conventionalsemiconductor saturable absorbers in that conventional semiconductorsaturable absorbers are engineered for each individual wavelength ofoperation, whereas a graphene saturable absorber does not need anyadditional engineering and operates in a similar fashion at allwavelengths from the visible through the infrared.

The combination of the SOA 140 with the graphene saturable absorber 130in a longitudinal or linear cavity arrangement also facilitatesmode-locking or Q-switching operation of the laser 100. The use of anSOA 140 as the gain medium for infrared wavelength advantageouslyfacilitates provision of optical gain at wavelengths ranging fromapproximately 230 nm to 14,000 nm by appropriate design of the quantumwell structure in certain embodiments of an SOA 140. Other optical gainmediums such as rare earth doped fiber amplifiers are not readilyavailable for operation at wavelengths longer than approximately 5,000nm, but a quantum cascade semiconductor optical amplifier (QCSOA) 140operates as an optical gain medium from approximately near infrared tofar infrared. In addition, the SOA 140 can be combined with the graphenesaturable absorber 130 in an external cavity arrangement to implement acompact mode-locked or Q-switched laser 100, wherein compact monolithicdesigns are also possible (e.g., FIGS. 1 and 12). In certainembodiments, for example, the graphene saturable absorber 130 can beintegrated with the SOA 140 to make an especially compact laser 100. Inaddition, the SOA 140 can be electrically pumped in certain embodimentsto facilitate creation of a compact laser device 100. Other embodiments(e.g., FIGS. 11 and 12) integrate the graphene saturable absorber 130with a VCSEL structure 170 to likewise facilitate implementation of acompact laser structure. Moreover, other optical gain mediums can beused, such as a rare earth doped fiber optical amplifier in combinationwith a graphene saturable absorber 130 to implement infrared mode-lockedor Q-switched laser device 100. In addition, while primarily illustratedin connection with electrically pumped SOAs 140 and other optical gainmediums 170, 180, graphene saturable absorbers 130 can also be used invarious embodiments in combination with optically pumped SOAs 140 andVCSELs 170. In certain embodiments, the laser 100 can be mode-locked orQ-switched for wavelengths within a wavelength band from 2.0 μm to 12μm.

The optical gain medium (whether an SOA 140, a semiconductor diode laseror vertical cavity semiconductor optical amplifier VCSOA 170, or otheroptical amplifier 180) can in certain embodiments be pumped by a secondlight source, such as another laser (e.g., 190 in FIG. 14 below). Forexample, infrared light can be amplified in the optical gain medium 140,170, 180 using an electrically driven solid-state laser. Alternatively,the gain media can be an optically pumped fiber, an optically pumpedcrystal, an optically pumped semiconductor optical amplifier in variousembodiments. A mode-locked laser can be implemented in a variety oflaser arrangements, for example a linear cavity (e.g., FIGS. 1-13), aring cavity (e.g., FIG. 15), and an extended semiconductor laser cavity(e.g., FIG. 14), including the optical gain medium within the opticalcavity. In each of these arrangements, the graphene saturable absorber130 can be used in transmission or reflection mode. The graphenesaturable absorber 130, due to its atomic layer thickness and itsability to be grown on one substrate then transferred to differentsubstrates, can be integral with one of the mirrors 110, 120 in certainembodiments for double pass use in reflection.

One factor in implementing a Q-switched laser device 100 is the storagetime for optically excited carriers in the optical amplifier medium 140,170, 180. In particular, if the optical amplifier storage time is shortit is difficult to make a Q-switched laser, however, it may be possibleto make a mode-locked laser. Semiconductor optical amplifiers designedfor infrared wavelengths such as Quantum Cascade SOA devices 140 orintersubband SOAs 140 typically only have a short storage time whichwould allow mode-locked laser operation but may not facilitateQ-switched laser operation. Semiconductor optical amplifiers 140designed for wavelengths less than 1,700 nm can have a storage time thatis sufficiently long to enable Q-switched operation. Mode-lockedoperation is also possible.

The embodiments of FIGS. 1-3 provide an integrated graphene saturableabsorber 130 and the SOA 140 in an optical cavity, where the cavityincludes a highly reflective mirror 110 on a first side 101, a graphenesaturable absorber 130 between the highly reflective mirror 110 and afirst facet end 141 of the SOA 140 with the graphene saturable absorber130 preferably monolithically integrated with the first facet 141. Anelectrically (or optionally optically) pumped SOA 140 optical gainmedium is operable at the selected wavelength and can also have narrowband or broad band operation, and a partially reflective mirror 120 canbe formed either on the second facet surface 142 of the SOA 140, or anoptional partially reflective mirror 120 can be spaced from the secondfacet 142 (FIG. 3) that performs as the output coupler of the laserdevice 100 on the second side 102.

The graphene saturable absorber 130 can be constructed so as toadvantageously provide optical absorption over a wide range ofwavelengths from ultraviolet (UV) to terahertz wavelengths, along with ahigh damage threshold and tunable response set in certain embodiments byselecting the number of graphene layers used in forming the saturableabsorber 130. In this regard, graphene films may be formed as sheets asthin as one atom thick, and the saturable absorption response can betuned/modified by increasing the number of layers of graphene. Graphenehas a high optical damage threshold, which can be particularlyadvantageous for Q-switched and mode-locked lasers 100 that produce peakirradiances greater than 10 GW/cm². Moreover, graphene material 130 hasa very high thermal conductivity, which is useful for removing absorbedheat in both the saturated and unsaturated cases.

In practice, the graphene saturable absorber 130 can operate in either atransmission mode or a reflection mode. In certain embodiments, thegraphene saturable absorber structure can be an antiresonant or resonantor no resonant design. A graphene saturable absorber 130 designed forthe transmission mode can optionally be combined with a partiallyreflective mirror 120 in certain embodiments. One design for a partiallyreflective second mirror 120 may include a Bragg mirror (also referredto as a dielectric mirror or distributed Bragg reflector) which can beany structure having an alternating sequence of layers of two or moredifferent optical materials, typically dielectric or semiconductorlayers, designed for specified reflectivity at different wavelengths oflight. A dielectric distributed Bragg reflector typically hasalternating layers of dielectric materials and a semiconductordistributed Bragg reflector typically has alternating layers ofsemiconductor materials. In other embodiments, a thin film coating orcoatings on a substrate can also be used to implement the partiallyreflective mirror 120, which may be integral with the second end 142 ofthe SOA 140 in certain implementations. The Bragg Reflector (orDistributed Bragg Reflector) may have reflectivity from a “periodicstructure” and not strictly reflection from a first reflective surface.

In one non-limiting example, the graphene saturable absorber 130 caninclude graphene material epitaxially grown on a silicon face of asilicon carbide (SiC) substrate. The graphene material typically growsapproximately one or several graphene sheet thick on the on-axissilicon-face of SiC. A Bragg material design or thin film deposited onthe SiC material or bonded to the SiC can implement a partially orhighly reflective mirror 110, where the example of FIG. 1 may beimplemented as a highly reflective first mirror 110 integrated with thegraphene saturable absorber 130. In other embodiments (e.g., FIGS. 11and 12 below), the graphene saturable absorber 130 can be integratedwith the second end 142 of the optical gain medium 140, 170, 180 and/orwith the partially reflective second mirror 120 by a variety offabrication techniques.

In other non-limiting embodiments, the graphene material 130 can beepitaxially grown on the carbon face of a SiC substrate, where suchgraphene material typically grows 10 graphene sheets to 40 graphenesheets thick. A partially or highly reflective mirror 110, 120 can beimplemented as discussed above for such embodiments.

In other implementations, the graphene saturable absorber material 130can be grown on a metal material and can then be transferred and bondedto a transparent substrate such as diamond substrate, sapphiresubstrate, quartz substrate, silicon substrate, SiC substrate, etc. ABragg mirror material design or thin film can be deposited on the SiCmaterial or bonded to the SiC in order to implement a partiallyreflective or highly reflective mirror 110, 120.

In further non-limiting embodiments, graphene flakes, graphenenanoplatelets, graphene nanosheets, graphene oxide flakes, grapheneoxide nanoplatelets, graphene oxide nanosheets, graphene material, orfluorographene material, etc can be deposited on a transparent substratesuch as a diamond substrate, a sapphire substrate, a quartz substrate, asilicon substrate, a partially reflective mirror substrate, reflectivemirror substrate, a substrate with a Bragg mirror, SOA first end, SOAsecond end, fiber optical amplifier first end, fiber optical amplifierssecond end, bulk crystal optical amplifier first end, bulk crystaloptical amplifier second end, etc., to provide the saturable absorber130. A Bragg material design or thin film deposited on the SiC materialor bonded to the SiC can implement a partially reflective or highlyreflective mirror 110, 120.

Still other possible non-limiting embodiments of the graphene saturableabsorber 130 can be created by incorporating the graphene material(including graphene flakes, graphene nanoplatelets, graphene nanosheets,graphene oxide flakes, graphene oxide nanoplatelets, graphene oxidenanosheets, graphene material, fluorographene material, etc.) into apolymer and then depositing or coating these on a transparent substrateor partially or highly reflective mirror 110, 120. A Bragg materialdesign or thin film deposited on the SiC material or bonded to the SiCcan implement a partially or highly reflective mirror 110, 120. In thisregard, diamond, boron nitride and SiC substrates advantageously providehigh thermal conductivity, and are thus attractive for applications thatwould require high power absorption in the graphene saturable absorber130. The silicon substrate, moreover, can be advantageous for infraredand terahertz application because of good transmission properties.

Using these and other fabrication techniques, the graphene saturableabsorber 130 can be designed to operate in the reflection mode bygrowing, transferring, bonding, depositing graphene or graphene oxidematerial on a substrate or on a first facet 141 of a SOA 140 or bydepositing a highly reflective coating on the graphene material 130. Onedesign for a highly reflective mirror 110 is to use a Bragg materialdesign mirror with a selected number of pairs. Thin film coating canalso be used to implement highly reflective mirrors. In addition, apartially reflective mirror (e.g., the second mirror 120 in FIG. 1) canbe implemented using the above-described or other techniques.

Referring now to FIGS. 2A and 2B, the semiconductor optical amplifier(SOA) in certain embodiments is preferably electrically pumped but canbe optically pumped via a pump 150 providing current or light 152 to theSOA structure 140 (e.g., FIG. 1). The SOA 140 provides the optical gainmedium that operates at the selected optical wavelength. The SOA 140 canhave narrow band or broadband operation. For example, broadbandoperation is preferred for operation of the optical cavity for frequencycomb spectroscopy for molecular sensing applications. The SOA 140 incertain embodiments may be a direct recombination SOA, quantum cascadeSOA, an intersubband SOA, ridge waveguide SOA designs, a vertical cavitysurface emitting SOA, and/or other SOA known to those skilled in theart. A quantum cascade or intersubband SOA design 140 can advantageouslygenerate optical amplified light at infrared wavelengths betweenapproximately 3 μm and approximately 24 μm wavelengths, and can generateterahertz wavelengths for special quantum cascade SOA designs. A directrecombination SOA 140 in certain embodiments can operate betweenapproximately 240 nm and approximately 3 μm wavelengths. Lead saltmaterial can also be used as semiconductor optical amplifier material.

In one non-limiting implementation, the highly reflective mirror 110 andthe graphene saturable absorber 130 can be integral with the first facetsurface 141 of the SOA 140. In certain embodiments, the first facet 141of the SOA 140 can be cleaved and/or coated for maximum transmission.The light from the first facet 141 is coupled out of the SOA 140. Ahighly reflective mirror 110 on a graphene material 130 can be used incertain embodiments to reflect the light back into the SOA 140, with thegraphene material 130 operating as a saturable absorber. The graphenesaturable absorber 130 can be attached, bonded or deposited onto the SOAfirst facet or end 141. There are several approaches by which thegraphene saturable absorber 130 can be integrated with the SOA 140. Oneapproach is to grow the graphene material 130 on a metal surface such ascopper or nickel (not shown). The front surface of the graphene material130 can be attached (bonded, glued, etc.) to the first facet 141 of theSOA 140. The metal material can then be optionally etched away leavingthe graphene material 130 on the SOA first facet 141. A highlyreflective mirror material 110 can then be deposited on the oppositeside of the graphene material 130. One option for implementing thehighly reflective material 110 on the graphene material 130 is to notetch away the metal material as discussed above, and simply use themetal as the reflective material 110. The graphene material 130 can alsobe deposited on the SOA facet 141 as graphene material flakes orgraphene nanoplatelets. The graphene material 130 can also beincorporated into a polymer and then deposited or coated on the SOAfacet 141. There is a minimal or no separation between the graphenematerial surface 130 and the SOA facet surface 141 in certainembodiments.

In the embodiment of FIG. 1, the second facet 142 or end of the SOA 140is cleaved and/or coated to provide the output coupler for the laserdevice 100, thus constituting the partially reflective second mirror 120with a partially reflective surface 122. As seen in FIG. 3, a separatepartially reflective mirror 120 can be used instead of relying on thesecond facet 142 to implement a partially reflective mirror of theresonant cavity. The linear cavity laser in the illustrated embodimentscan optionally include a region for sensing within the optical cavity,for example, for insertion of gas samples to be measured to determinethe gas molecules within the region for sensing. The linear cavity laser100 can be operated in certain embodiments in an optical frequencyspectroscopy mode (e.g. frequency-comb spectroscopy) to permit enhancedsensing of the gas molecules within such region for sensing.

The disclosed laser devices 100 (and 200 in FIG. 15) in variousembodiments provide certain advantages by use of a graphene saturableabsorber 130, particularly for mode-locked or Q-switched operation. Forexample, the disclosed devices 100, 200 facilitate mode-locked operationat infrared wavelengths with center wavelengths in the range of 1,800 nmto 25,000 nm, with a spectral width as much as 10% or more of the centerwavelength. In addition, certain non-limiting embodiments facilitatecompact Q-switched or mode-locked laser structures operable with centerwavelengths in the range of 280 nm to 1,800 nm and having a spectralwidth as much as 10% or more of the center wavelength. Variousembodiments facilitate implementation of a high repetition, femtosecondlaser at infrared wavelengths. Moreover, certain embodiments facilitateimplementation of Frequency Comb Spectroscopy (which is a highlysensitive technique for performing molecular sensing) at infraredwavelengths, with comb frequency spread over a >1000 nm spectral width.Also, the disclosed devices 100, 200 can provide a compact laser bycombining a graphene saturable absorber 130 with a semiconductor opticalamplifier 140 in an external cavity. In this regard, the disclosed laserdevices 100, 200 can be implemented as a compact laser by integrating agraphene saturable absorber 130 with a semiconductor optical amplifier140 in an external cavity. Certain embodiments, moreover, provide acompact laser 100, 200 by integrating a graphene saturable absorber intoa VCSEL structure (e.g., FIGS. 11 and 12 below).

FIG. 4 illustrates another embodiment in which the second facet 142 ofthe SOA 140 can be cleaved and/or coated for maximum transmission, and apartially reflective mirror 120 performs as the output coupler of thelaser 100. In addition, an optional gas cell or other optical component160 can be located between the second facet 142 of the SOA 140 and thepartially reflective mirror 120. For operation at infrared wavelengthslonger than approximately 1,700 nm, one particularly advantageousoptical cavity arrangement is as a linear optical cavity, particularlyfor situations in which optical isolators are not available to implementa ring cavity design. The optical cavity arrangement is also appropriatefor lasers 100 that operate at center wavelengths less than 1,700 nm.

Referring also to FIGS. 5-9, certain embodiments may employ the graphenesaturable absorber 130 spaced from the first end 141 of the SOA 140,where some embodiments (e.g., FIGS. 6 and 8) may also provide one ormore optical components 160, such as a gas cell in certainimplementations, disposed between the graphene saturable absorber 130and the first SOA facet end 141.

Some embodiments, moreover, may employ one or more optical components160 (e.g., a gas cell in certain implementations) disposed at leastpartially between the second SOA facet end 142 and the second mirror120, as seen in FIG. 9. In certain implementations, moreover, thegraphene saturable absorber 130 may be integral with the first mirror110. As seen in FIG. 5, for example, this provides a structure includinga graphene saturable absorber 130 in combination with an SOA 140 in anextended optical cavity of the device 100. As seen in FIGS. 5-9,moreover, the optical cavity includes a highly reflective mirror 110 ona first side 101, a graphene saturable absorber 130 between the highlyreflective mirror 110 and a first facet 141 of the SOA 140 (in onearrangement, the graphene saturable absorber 130 can be integrated withthe mirror 110).

In addition, as seen in FIGS. 6 and 8, an optional gas cell or otheroptical component 160 can be disposed at least partially between thehighly reflective mirror 110 and the SOA first facet 141. The SOA 140can be electrically or optically pumped, and can operate at the selectedwavelength for narrow band or broadband operation. In addition, apartially reflective mirror 120 can be formed either on the SOA secondfacet surface 142 or as a separate partially reflective mirror structure120 separated from the second facet 142 that operates as the outputcoupler of the resulting laser device 100 on the second side of theoptical cavity 102.

FIG. 10 illustrates another non-limiting embodiment of the laser device100, in which the graphene saturable absorber material 130 is integratedwith the first facet 141 of the SOA 140. In certain implementations, atleast one optical component 160 is longitudinally disposed between thegraphene saturable absorber 130 and the first mirror 110 as seen in FIG.10, and other implementations are possible in which no optical component160 is provided in the device 100. In certain embodiments, moreover, thegraphene material 130 can be integrated with the SOA 140, for example,with graphene being deposited or bonded to the first facet surface 141in certain implementations.

Referring also to FIGS. 11, 12 and 18-20, FIG. 11 illustrates a linearcavity (external cavity) laser device 100 including a graphene saturableabsorber 130 and an integrated partially reflective second mirror 120,where the graphene saturable absorber 130 may be spaced from a secondfacet end 172 of the optical gain medium 170 as shown in FIG. 11, or mayinstead be integrated with the optical gain medium 170 as shown in FIG.12. As with the above embodiments, the first and second mirrors 110 and120 define a resonant optical cavity to establish a standing wave (or atraveling wave) of electromagnetic energy and to facilitate emission ofstimulated coherent radiation through the second mirror 120. The mirror110 can be a Distributed Bragg Reflector (DBR), a DBR in combinationwith a metal reflector, or a metal reflector. The optical gain medium inthe embodiments of FIGS. 11 and 12 is a semiconductor optical amplifier170 with a first facet end 171 and a second facet end 172 disposed atleast partially between the first and second reflective surfaces 112,122 of the mirrors 110, 120, respectively. The second facet end 172 canbe coated with antireflecting material or antireflection materials toreduce the reflection of light back into the semiconductor opticalamplifier 170. The second facet end can also be tilted to reduce thereflection of light back into the semiconductor optical amplifier. Anelectrical or optical pump 150 is operable to provide electrical currentand/or light to the semiconductor optical amplifier 170.

In these embodiments, the graphene saturable absorber 130 is integralwith the second mirror at least partially between the semiconductoroptical amplifier 170 and the second reflective mirror surface 122. Anyform of semiconductor optical amplifier 170 can be used in theseembodiments. For instance, the semiconductor optical amplifier can be avertical cavity surface emitter optical amplifier (VCSOA) 170 disposedat least partially between the first and second reflective surfaces 112,122. In the embodiment of FIG. 12, moreover, the graphene saturableabsorber 130 can be bonded to or deposited on the second end 172 of thesemiconductor optical amplifier 170 with optional spacer layer betweenthe graphene saturable absorber and the second end 172. The process ofbonding the graphene saturable absorber to the second end 172 can be aprocess or direct bonding, adhesive bonding, or other bonding approach.The second end 172 can be the surface of the vertical cavitysemiconductor optical amplifiers gain region and can also be the surfaceof the semiconductor. Additional material such as semiconductor materialor dielectric material can be deposited or bonded to the surface of thegraphene saturable absorber 130 to act as a phase shift layer or spacerlayer. The second mirror 120 can be integral with or onto the graphenesaturable absorber 130 to provide a monolithic laser structure.

In certain embodiments, the second mirror 120 can be a partiallyreflecting distributed Bragg reflector. In certain embodiments, thegraphene saturable absorber can be integral with the second mirror 120with an optical spacer material, and then the graphene saturableabsorber and second mirror can be bonded to the second end 172 or to aspacer material layer on the surface of the second end 172. In certainembodiments, the graphene saturable absorber 130 can be bonded to thesecond end 172 or to a spacer material on the surface of the second endand then a distributed Bragg reflector can be grown on the surface ofthe graphene saturable absorber or on the surface of a spacer materialdeposited on the surface of the graphene saturable absorber 130. Theseparation between the second mirror 120 and the second end 172 can becontrolled by a MEMS device to change the length of the cavity. Themirror 110 can be a Distributed Bragg Reflector (DBR), a DBR incombination with a metal reflector, or a metal reflector.

As seen in FIGS. 18-20, a graphene saturable absorber 130 can optionallyfunction as, and/or form a part of, a top electrode for the VCSOA, andmay be a transparent electrode for the VCSOA. A metal layer used to formmirror 110 or a metal layer on the bottom surface of the DBR that formsmirror 110 can function as the bottom electrode for the VCSOA. In FIG.18, a graphene layer 130 is formed on the top side of the SOA 170 tofunction as a transparent top electrode for the VCSOA. In FIG. 19, anoptional spacer material layer 131 can be formed on the top side of theSOA 170, with the graphene layer 130 being formed above the spacermaterial layer 131. As seen in FIG. 20, moreover, the vertical cavitydevice can include a metal structure or layer 132 formed over thegraphene 130 (with or without the optional spacer layer 131). In theexample of FIG. 20, moreover, the top metal electrode structure 132includes an opening 133 exposing at least a portion of the graphenelayer 130. Various different implementations are possible in which a topelectrode is formed above (directly or with intervening structures ormaterials) a top surface of the SOA 170, where the top electrode caninclude a graphene material layer, preferably transparent. In thisregard, the graphene material layer 130 forming at least a part of thetop electrode structure can operate as a saturable absorber. In certainimplementations (e.g., FIGS. 19 and 20), a spacer material layer 131 isprovided between the upper surface of the SOA 170 and the graphenematerial layer 130. Certain embodiments of the top electrode, moreover,can include a metal structure 132 formed (directly or indirectly) overthe graphene 130. In addition, such a metal structure 132 can be a ringstructure with a cylindrical opening 133 as shown in FIG. 20, or can bea solid layer, or can include an opening or aperture 133 of any suitablesize and shape which exposes at least a portion of the graphene materiallayer 130. The vertical current flow in the VCSOA, in this regard, canbe confined by the design of the top metal electrode 132 and/or byoxygen ion implant or by other approaches known to those skilled in theart. The top metal electrode 132 can be a ring shape with an opening 133in the center for light emission. The top metal electrode 132 cancontact the graphene saturable absorber 130 which can function as atransparent electrode to facilitate a uniform vertical current flowand/or larger light emission area. Current flow confinement approachessuch as oxygen ion implantation or proton ion implantation can be usedto aid in confining the vertical current flow to the active region ofthe vertical cavity semiconductor optical amplifier (VCSOA) or verticalcavity surface emitting laser (VCSEL) in the case that the second mirroris integral to the VCSOA with integral graphene saturable absorber orVECSEL for the case that the second mirror is separated from the VCSOA.

In one possible implementation, the partially reflective mirror 120 andthe graphene saturable absorber 130 are integral with the samesubstrate. The second facet 172 of the solid-state gain medium 170 iscleaved or ion milled (optionally at a tilt angle) and/or coated formaximum transmission and/or minimum reflection in certain nonlimitingembodiments. The light from the second facet 172 is coupled out of thegain medium 170, and a partially reflective mirror 120 on a graphenematerial 130 is used to reflect the light back into the optical gainmedium 170. The graphene material 130 performs as a saturable absorber,and can be attached, bonded or deposited onto the second facet 172 ofthe optical gain medium 170, and/or to the partially reflective mirrorsubstrate 120 or onto a separate transparent substrate (not shown). Thepartially reflective mirror 120 can be implemented in certainembodiments as a Bragg design or maybe implemented as thin filmmaterial.

There are several approaches by which the graphene saturable absorber130 can be integrated with the partially reflective mirror 120 or thegain medium second facet 172. One non-limiting approach is that thegraphene material 130 can be first grown on a metal surface (not shown)such as copper or nickel. The front surface of the graphene material 130can be attached (e.g., bonded, glued, etc.) to the mirror substrate 120and/or to the second facet 172. The metal material can then beoptionally etched away leaving the graphene material as a transparentmaterial on the mirror substrate. A partially reflective mirror material120 can then be deposited on the other side of the graphene material130. The graphene material 130 can also be deposited on the partiallyreflective mirror substrate 120 or second optical gain medium facet 172as graphene material flakes or graphene nanoplatelets and certainnon-limiting implementations. The graphene material 130 can also beincorporated into a polymer and then deposited or coated on thepartially reflective mirror 120 or second facet 172 of the optical gainmedium 170. The linear cavity laser 100 in certain embodiments canoptionally include a region for sensing within the optical cavity forinsertion of gas samples to be measured to determine the gas moleculeswithin the sensing region. In addition, the linear cavity laser 100 canbe operated in the Frequency Comb Spectroscopy mode to permit enhancedsensing of the gas molecules within such sensing region.

In these embodiments of FIGS. 11 and 12, moreover, the first mirror 110is integrated with the first SOA facet 171, or can be a Bragg mirrorintegrated within the optical gain medium 170. Moreover, an electricallyor optically pumped gain medium 170 operates in certain embodiments atthe selected wavelength and can also have narrow band or broadbandoperation. In the implementation of FIG. 12, the graphene saturableabsorber material 130 can be bonded to the second facet 172 of the gainmedium 170, and the partially reflective mirror 120 can be integratedwith the graphene saturable absorber 130 to implement a monolithic ornear monolithic laser structure 100. In one non-limiting implementation,the partially reflective mirror 120 and the graphene saturable absorber130 can be integral with the same substrate. The second facet 172 of thesolid-state gain medium 170 may be cleaved and/or coated for maximumtransmission in certain embodiments, such that the light from the secondfacet 172 is coupled out of the gain medium 170. The partiallyreflective mirror 120 may be integrated with the graphene material 130so as to reflect the light back into the gain medium 170. The graphenematerial 130 performs as a saturable absorber, and can be attached,bonded or deposited onto the second facet 172, or onto the partiallyreflective mirror substrate 120 or onto a separate substrate. Thepartially reflective mirror 120 in certain embodiments can beimplemented as a Bragg design or may be implemented by thin filmmaterial.

There are several approaches by which the graphene saturable absorber130 can be integrated with the partially reflective mirror 120 or thesecond facet 172. For example, the graphene material 130 can be firstgrown on a metal surface such as copper or nickel, and the front surfaceof the graphene material 130 can be attached (bonded, glued, etc.) tothe mirror substrate 120 or the second facet 172. The metal material canthen be optionally etched away leaving the graphene material 130 on thefacet 172. A highly reflective mirror material can then be deposited onthe other surface of the graphene 130. The graphene material 130 canalso be deposited on the partially reflective mirror substrate 120 orthe second facet 172 as graphene material flakes or graphenenanoplatelets. The graphene material 130 can also be incorporated into apolymer and then deposited or coated on the partially reflective mirrorsubstrate 120 or second facet 172. The linear cavity laser 100 can alsoinclude a region for sensing within the optical cavity for inserting gassamples to be measured to determine the gas molecules within the sensingregion. In addition, the linear cavity laser can be operated in theFrequency Comb Spectroscopy mode to permit enhanced sensing of the gasmolecules within the sensing region.

Referring also to FIGS. 13 and 14, a linear cavity laser device 100 isprovided with a first mirror 110 with a first reflective surface 112 anda second mirror 120 with a second reflective surface 122 which at leastpartially faces the first reflective surface 112. The first and secondmirrors 110 and 120 define a resonant optical cavity in whichelectromagnetic energy is amplified by stimulated emission of coherentradiation that is partially transmitted through the second mirror 120. Asolid-state optically pumped infrared optical gain medium 180 isdisposed at least partially between the first and second reflectivesurfaces 112 and 122, and is pumped via an optical pump 150 providinglight to the optical gain medium 180. In addition, a graphene saturableabsorber 130 is disposed at least partially between the solid-stateoptically pumped infrared optical gain medium 180 and the firstreflective surface 112. The gain medium 180 in certain embodiments canbe a rare earth doped fiber amplifier operable at wavelengths of about1400 nm or more and about 3000 nm or less. In other embodiments, thesolid-state gain medium 180 includes at least one II-VI family dopedcrystal. Further embodiments provide an optical gain medium 180 thatincludes at least one transition metal doped zinc chalcogenide, or atleast one lead salt. Moreover, various embodiments may include one ormore optical components 160 (e.g., a gas cell) longitudinally disposedbetween the solid-state optical gain medium 180 and the second mirror120 as shown in FIGS. 13 and 14, or such may be omitted in otherimplementations.

In these embodiments, the resonator or laser optical cavity is createdwith a solid-state optically pumped gain medium 180 within the opticalcavity. In certain embodiments, the solid-state optically pumped opticalgain medium can be an infrared (including near infrared or mid infrared)solid-state optically pumped optical gain medium. Typical types ofsolid-state optical gain mediums 180 include crystals, glasses,ceramics, or composite laser material doped with rare earth ions, dopedwith transition metal ions, or semiconductor gain medium. The ion dopedsolid-state laser 100 can be made in the form of bulk laser, fiber laseror other types of waveguide lasers. The solid-state laser 100 maygenerate output power at levels between a few milliwatts and manykilowatts.

The solid-state optically pumped laser embodiments 100 may be either endpumped or side pumped in certain embodiments. The solid-state opticallypumped gain medium 180 may have the geometry of a fiber, rod, slab,microchip, etc. Many solid-state lasers are optically pumped with flashlamps, arc lamps, or laser diodes. The laser transitions of rare-earthdoped or transition metal doped solid-state optically pumped gainmediums 180 are normally weakly allowed transitions; i.e. transitionswith very low oscillator strength, which leads to long radiativeupper-state lifetime and consequently yields good energy storage withlong upper-state lifetimes of microseconds to milliseconds and is thusadvantageous for Q-switched lasers 100. The laser crystal can easilystore an amount of energy which, when released in the form of ananosecond pulse, leads to a peak power which is orders of magnitudeabove the average power.

In mode-locked operation, the solid-state laser 100 can generateultra-short pulses with durations on the order of picoseconds orfemtoseconds. Passive mode-locking can have Q-switching instabilitieswhich are suppressed via suitable design and operational control,including parameters such pump energy, energy storage lifetime, cavitylength, etc. Most rare-earth doped laser crystals, such as Nd:YAG andNdYVO₄ have small gain bandwidths so that tuning is possibly only withina rather limited range. Tuning ranges of tens of nms and more arepossible with rare-earth-doped glasses, and particularly withtransition-metal doped crystals such as Ti:sapphire, Cr:LiSAF andCr:ZnSe (vibronic lasers).

In some solid-state gain mediums 180, particularly in those doped withtransition metal ions, there is a strong interaction of the electronicstates with lattice vibrations (phonons). This vibrational-electronic(vibronic) interaction leads to a strong homogeneous broadening and thusa large gain bandwidth. Examples of solid-state gain medium materialinclude rare earth doped fiber amplifier such as erbium-doped fiberamplifiers that operate at wavelengths from 1500 to 1600 nm. Otherexamples include doped crystals of the II-VI family, for example,Cr₂+:ZnSe chromium doped II-VI compound based lasers as the mostsuccessful room-temperature diode-pumpable sources in the wave-lengthrange between about 2 μm and about 3.5 μm; Cr:CdSe; Fe₂+:ZnSe lasers canemit at wavelengths of about 3.7 μm to 5.1 μm; and Cr₂+:CdMnTe. Otherpossible implementations can employ transition metal-doped zincchalcogenides, such as Tm:YAL, Tm:YAG, Rare earth doped or transitionmetal doped laser ceramics, and/or lead salt such as PbSSe, PbSnTe,PbSnSeTe and/or PbSnS.

The infrared gain material 180 preferably absorbs light at one or morewavelengths (e.g., from the optical pump 150) and amplifies light in thedesired infrared 2000 to 6000 nm optical band. The optically pumpedinfrared gain material 180 performs the functions of a gain medium thatoperates at the operational wavelength, and creates infrared lightwithin the optical cavity which can be amplified. In this linear lasercavity, the optically pumped infrared gain material 180 is locatedbetween two mirrors 110 and 120 which provide the feedback for the laserdevice 100. In addition a graphene saturable absorber 130 is placedwithin the cavity. The location of the graphene saturable absorber 130in certain embodiments will modify the performance of the laser device100 to produce the desired mode-locked laser pulses.

Optional optical devices 160 such as lenses, folding mirrors, spectralfilters, polarization and dispersive optics can be inserted between themirrors 110, 120 in order to improve the device performance. The laseroutput can be transmitted through one or more of the mirrors 110, 120,e.g., from a beam splitter, or from an intra-cavity reflection. Afurther variation can include a highly reflective mirror (e.g., mirror110) on a first side of the optical cavity, a graphene saturableabsorber 130 between the highly reflective mirror 110 and a first facet181 of the optical amplifier material 180 (in one arrangement, thegraphene saturable absorber 130 can be integrated with the mirror 110),an optional gas cell 160 located between the mirror 110 and the facet181 of the optical gain medium 180, an optically pumped optical gainmedium 180 that operates at the selected wavelength and can also havenarrow band or broad band operation, and a partially reflective mirror120 separated from the second facet 182 of the gain medium 180 whichperforms as the output coupler of laser device 100 on the second side102 of the optical cavity. The linear cavity laser 100 may include asensing region within the optical cavity for inserting gas samples to bemeasured to determine the gas molecules within the sensing region, andthe laser device 100 can be operated in the Frequency Comb Spectroscopymode to permit enhanced sensing of the gas molecules within sensingregion.

FIG. 14 illustrates a linear cavity laser device 100 with a graphenesaturable absorber 130 and two partially reflective mirrors 120A and120B, respectively, forming an injection locked (coupled) optical cavitywith a laser 190 to direct light at least partially toward a second sideof the first partially reflective mirror 120A. An optical gain medium180 is disposed at least partially between the first and secondreflective surfaces 112, 122 of the respective mirrors 120A and 120B,and is pumped by an electrical or optical pump 150 that provideselectrical current or light to the gain medium 180. A graphene saturableabsorber 130 is disposed at least partially between the optical gainmedium 180 and the first reflective surface 112, and the laser 190directs light at least partially toward the first partially reflectivemirror 120A. In certain embodiments, one or more optical components 160can be longitudinally disposed between the optical gain medium 180 andthe second mirror 120, for example, a gas cell. The saturable absorber130 in this embodiment, as with the other embodiments illustrated anddescribed herein, can be a graphene material or a graphene-containingmaterial, and can be integrated with the partially reflective mirror 110as shown. The mirror 110, moreover, can include a mirror coating on thegraphene material 130, or the graphene material 130 can be formed on amirror structure 110, or the partially reflective mirror 110 can beformed on a separate substrate in various embodiments.

FIG. 15 shows a ring cavity laser device 200 in accordance with furtheraspects of the present disclosure, which includes two highly reflectivemirrors 110A and 110B with reflective surfaces 112A and 112B,respectively, along with a partially reflective mirror 120 providing anoutput coupling for the device 200. An optical amplifier or gain medium180 is disposed between the mirror 110A and the partially reflectivemirror 120, and a graphene saturable absorber 130 is disposed at leastpartially between the second highly reflective mirror 110B and thepartially reflective mirror 120. In this configuration, a reflectivesurface 112B of the mirror 110B at least partially faces the reflectivesurfaces 112A and 122 of the other mirrors, and the same is true of theother mirrors, whereby the reflective surface of each mirror at leastpartially faces that of the other mirrors. Moreover, in thisconfiguration, the mirrors 110A, 120 and 110B define an optical ringcavity to establish a standing wave of electromagnetic energy and tofacilitate emission of stimulated coherent radiation through the secondmirror 120. An electrical or optical pump 150 provides current or lightto the optical gain medium 180, and the graphene material 130 operatesas a graphene saturable absorber 130 disposed at least partially betweenthe second and third reflective surfaces 122 and 112B. Variousembodiments are possible, wherein the graphene saturable absorber 130can operate in the transmission mode in certain embodiments. The gainmedium 180 in certain embodiments can be an optical pumped fiber orcrystal gain medium, or an electrically biased (electrically pumped)semiconductor optical amplifier (SOA).

In various nonlimiting implementations, the optical gain medium 180 canbe contacted by one or more of the reflective surfaces 112, 122 and/orby an optical element 160.

Referring now to FIGS. 16 and 17, a linear cavity laser device and/or aring cavity laser device may include more than two mirrors and at leastone graphene saturable absorber. In this regard, the device shown inFIG. 16 is linear cavity laser device with four mirrors M1-M4 and twooptical elements OE1 and OE2, in which at least one of the opticalelements OE1 and/or OE2 is a graphene saturable absorber. FIG. 17 showsan exemplary ring cavity laser device with five mirrors M1-M5 and fouroptical elements OE1-OE4, where at least one of the optical elementsOE1-OE4 is a graphene saturable absorber.

In certain nonlimiting implementations, a bandpass filter component canbe provided within the cavity. In certain embodiments, an etalon can beprovided in combination with a spectral filter to establish multiwavelength operation. In addition, the facet on the output end may be ananti-reflection coating, and the facet may be titled in certainembodiments to prevent or mitigate feedback (e.g., minimize or reduceFabry-Perot resonances) due to the residual facet reflectivity.Moreover, the output coupler in certain embodiments can be an externalmirror with partial reflectivity. In certain embodiments, themode-locked laser can be implemented using passive mode locking, butembodiments are contemplated in which hybrid mode locking is used (takesadvantage of both the stability offered by an actively mode-lockedsystem and the pulse shortening mechanisms provided by the saturableabsorber (SA)). For instance, a hybrid mode-locked (HML) monolithicallyintegrated indium phosphide (InP) laser can be used, where the hybridmode-locking scheme takes advantage of both the stability offered by anactively mode-locked system and the pulse shortening mechanisms providedby the graphene saturable absorber (SA). In other embodiments, a hybridSOA-Raman amplifier can be used in conjunction with a graphene saturableabsorber.

In certain embodiments, moreover, the emission can be out of the bottomof the substrate (e.g., the light goes horizontal and then vertical nearthe end). Quantum Cascade Amplifiers, for example, can have an emissionout of the bottom substrate, and the graphene saturable absorber may bebonded to the back surface of the substrate. In a substrate emitting DFBQCL configuration, the functions of the distributed feedback and thesurface emission may be separated.

Moreover, various embodiments can employ frequency tuning by varying thecurrent (e.g., electrically pumped embodiments). In various embodiments,a Quantum Cascade Amplifier can be combined with a Bragg region. Strongfeedback needed for single-mode operation may be obtained by afirst-order Bragg grating. A dispersive element can be used in certainembodiments, for example a diffraction grating in a laser with anexternal cavity in order to spectrally filter the radiation reflected inthe laser amplifier in order to produce globally single-mode laserradiation. Another example of a dispersive element implementationinvolves replacing one or more of the Bragg mirrors with a chirped Braggmirror, thereby altering the spectral phase of the light as it isreflected by the applied chirp. One possible design for a QuantumCascade Amplifier is a Master Oscillator Power Amplifier (MOPA) in whicha Bragg region is used to select out a signal mode. For suchembodiments, the graphene saturable absorber can be located between theQuantum Cascade output facet and the external mirror.

The above examples are merely illustrative of several possibleembodiments of various aspects of the present disclosure, whereinequivalent alterations and/or modifications will occur to others skilledin the art upon reading and understanding this specification and theannexed drawings. In addition, although a particular feature of thedisclosure may have been illustrated and/or described with respect toonly one of several implementations, such feature may be combined withone or more other features of the other implementations as may bedesired and advantageous for any given or particular application. Also,to the extent that the terms “including”, “includes”, “having”, “has”,“with”, or variants thereof are used in the detailed description and/orin the claims, such terms are intended to be inclusive in a mannersimilar to the term “comprising”.

The following is claimed:
 1. A laser device, comprising: a first mirrorwith a first surface; a second mirror with a second surface at leastpartially facing the first surface of the first mirror, the first andsecond mirrors defining a resonant optical cavity in whichelectromagnetic energy is amplified by stimulated emission of coherentradiation that is partially transmitted through the second mirror; asemiconductor optical amplifier disposed at least partially between thefirst and second surfaces; wherein the semiconductor optical amplifiergain exceeds the loss within the resonant optical cavity over a broadspectral bandwidth; an electrical or optical pump providing energy tothe semiconductor optical amplifier; and a graphene saturable absorberdisposed at least partially between the first and second surfaces;wherein the graphene saturable absorber operates in a passive mode;wherein the laser device emits stimulated coherent radiation through thesecond mirror with center wavelengths of about 1,800 nm or more andabout 25,000 nm or less; and has a spectral width of about 0.006% ormore of a center wavelength.
 2. The laser device of claim 1, wherein thelaser device is a mode-locked laser or a Q-switched laser configured toemit stimulated coherent radiation through the second mirror with centerwavelengths of about 280 nm or more and about 1,800 nm or less.
 3. Thelaser device of claim 1, wherein the semiconductor optical amplifiercomprises a superlattice of alternating semiconductor layers of at leasttwo different material compositions.
 4. The laser device of claim 1,wherein the graphene saturable absorber is disposed between the firstsurface and the semiconductor optical amplifier.
 5. The laser device ofclaim 4, wherein the semiconductor optical amplifier comprises oppositefirst and second ends, and wherein the graphene saturable absorber isintegral with the first end of the semiconductor optical amplifier. 6.The laser device of claim 5, wherein the second mirror is integral withthe second end of the semiconductor optical amplifier.
 7. The laserdevice of claim 6, wherein the second mirror is a partially reflectivefacet on the second end of the semiconductor optical amplifier.
 8. Thelaser device of claim 6, wherein the second mirror is a partiallyreflective coating on the second end of the semiconductor opticalamplifier.
 9. The laser device of claim 5, wherein the second mirror isspaced from the second end of the semiconductor optical amplifier. 10.The laser device of claim 9, wherein the second mirror is a Braggmirror.
 11. The laser device of claim 9, further comprising at least oneoptical component disposed between the second end of the semiconductoroptical amplifier and the second mirror.
 12. The laser device of claim11, wherein the at least one optical component is a gas cell.
 13. Thelaser device of claim 4, wherein the graphene saturable absorber isintegral to the first mirror.
 14. The laser device of claim 13, whereinthe first mirror is formed as a highly reflective coating on thegraphene saturable absorber.
 15. The laser device of claim 13, whereinthe first mirror is a Bragg mirror or a highly reflective coating on asubstrate.
 16. The laser device of claim 4, wherein the graphenesaturable absorber is spaced from the semiconductor optical amplifier.17. The laser device of claim 16, wherein the graphene saturableabsorber is integral to the first mirror.
 18. The laser device of claim16, further comprising at least one optical component disposed betweenthe first end of the semiconductor optical amplifier and the graphenesaturable absorber.
 19. The laser device of claim 18, wherein the atleast one optical component is a gas cell.
 20. The laser device of claim16, further comprising at least one optical component disposed betweenthe second end of the semiconductor optical amplifier and the secondmirror.
 21. The laser device of claim 20, wherein the at least oneoptical component is a gas cell.
 22. The laser device of claim 4,wherein the graphene saturable absorber is spaced from the first mirror,further comprising at least one optical component disposed between thegraphene saturable absorber and the first mirror.
 23. The laser deviceof claim 22, wherein the at least one optical component is a gas cell.24. The laser device of claim 22, wherein the graphene saturableabsorber is integral with the semiconductor optical amplifier.
 25. Thelaser device of claim 1, wherein the semiconductor optical amplifier isa quantum cascade semiconductor optical amplifier.
 26. The laser deviceof claim 1, wherein the semiconductor optical amplifier is an interbandcascade.
 27. The laser device of claim 1, wherein the laser device is afrequency comb laser.
 28. The laser device of claim 1, wherein the laserdevice is a mode-locked laser or a Q-switched laser configured to emitstimulated coherent radiation through the second mirror and having aspectral width of about 10% of a center wavelength.
 29. The laser deviceof claim 1, wherein the laser device is a linear cavity or externalcavity mode-locked laser or a Q-switched laser configured to emitstimulated coherent radiation through the second mirror.
 30. The laserdevice of claim 1, having a spectral width of about 0.5% or more of acenter wavelength.
 31. The laser device of claim 30, having a spectralwidth of about 2.0% or more of the center wavelength.
 32. The laserdevice of claim 31, having a spectral width of about 10.0% or less ofthe center wavelength.